This invention relates to a method of manufacturing a wheel rim for a two-piece vehicle wheel assembly. The invention is especially applicable to the automobile parts industry.
One of the largest aftermarket niches in the automobile industry is the manufacture and sale of upgraded alloy wheels. Wheels are a major styling feature which can quickly freshen the appearance of older vehicles, or customize the appearance of new vehicles still at the dealership. Traditionally, aluminum alloy wheels have been cast in one-piece with integrally-formed rim and center sections.
The one-piece wheel was first introduced in the industry as an aftermarket product about 40 years ago. While this wheel was and still is largely successful, problems in manufacturing make it difficult to obtain a high quality casting and finished wheel on a consistent basis. Various manufacturing techniques have been tried in an effort to make a better product, including making the rim section thicker to keep it from leaking, and to increase its overall strength and toughness.
In more recent years, the two-piece alloy wheel has become a viable alternative to one-piece wheels. The two-piece wheel includes an annular wheel rim or "hoop", and a center. The center is cast and machined as before, and then welded to the separately formed rim. The rim is generally formed using a non-heat treatable aluminum alloy sheet which is cut in a strip by shearing to the desired length and width. The strip is passed through a three-roll bender to form a hoop. The butted ends of the hoop are then flattened out and clamped together in a massive electric-resistance upset butt welder. The upset flash at the weld is broached off, and the joint finely sanded to make it non-prominent. The hoop is then flared along each side edge, and is loaded into either a rim rolling or rim spinning line. Rolling basically bends the edge of the rim axially around a mandrel keeping the thickness generally constant. Spinning, on the other hand, pushes the rim axially on the mandrel causing cross-sectional variation. The latter technique reduces the weight and improves the concentricity and strength of the rim. Finally, the wheel assembly is formed by attaching the finished rim to a selected wheel center.
This conventional approach using alloy sheet strips produces quality rims and, as the wheel center is a simple casting which is not required to seal-in air, the two-piece wheel quickly found a market in the industry. Sheet rims are generally made in large quantities for cost effectiveness, and are used with different style wheel centers and with various offsets to allow wide variation in wheel design. Despite this growing market, the one-piece cast wheel community persisted and continued to focus on improving difficult wheel casting methods. There was little incentive at this time to look into casting and machining rims as an alternative to sheet-formed rims in a two-piece wheel assembly. However, because sheet rims are made from sheet, the rims are only 21/2 D (dimensional) shapes, i.e., they do not have the 3D shape capability that cast wheels possess in the rim area. Due in part to this limitation, there grew a demand in the industry for cast wheel rims.
Around 1996, the industry experimented with a process for casting and machining wheel rims with a rounded street side flange. The two-piece all-cast wheels made using this process did not sell well, and because they were more expensive to make than either a traditional two-piece or similar looking one-piece wheel, these earlier cast and machined alloy rims were discontinued. They did however suggest an approach for combining the advantages of both one-piece and two-piece wheels.
In the process of developing a cast wheel rim, it was determined that a 100% cast and machined rim was dimensionally truer than conventional sheet rims used in two-piece wheel assemblies, thereby resulting in a smoother vehicle ride. The key was to develop a more cost efficient process for manufacturing the cast wheel rim.
One manufacturer's current approach is to cast and heat treat a one-piece wheel form as normal, followed by cutting out the spoked center with a plasma torch, followed by conventional wheel machining. While the casting is designed with a functional "center", its only purpose is to deliver the molten metal to the rim, and not to carry loads. The plasma cutting results in a cast rim shape, i.e., a convoluted hoop, which must then be machined completely to form the rim. Machining typically consists of two operations. In the first operation, the cast front side of the rim is centered and clamped by a dedicated first operation chuck, after which the back side of the rim and wheel is machined to final dimension. In the second operation, the machined inner portion of the wheel rim and center is used to precisely locate and clamp the rim in a separate chuck so that the front side of the wheel can be finish machined to blend perfectly to the first operation machined rim diameters.
Although this process produces high quality rims, it is problematic in several respects. Because the rim casting is essentially a large diameter thin walled hoop, it is very difficult to adjust the machining lathe so as to avoid excessive chatter marks (vibration induced surface irregularities) on the machined surfaces. Other drawbacks to this process include excessive scrap, machining cycles running at twice the rate as those for one-piece wheels, excessive technician set-up time, low cutting insert life, and an extra sanding step required to smooth out the course machining and/or chatter marks in the subsequent rim polishing process. Generally, although various remedies have been explored and minor improvements made, this process is only half as efficient as normal casting and machining for one-piece wheels. Nonetheless, this process is still being used today.
To overcome these and other limitations of the prior art, Applicant determined that a higher quality wheel rim could be produced by leaving the cast center and rim sections of the wheel form in one integrally-formed piece during the entire machining process. The first operation machining of the wheel form and turning was performed in a conventional manner--a well known and essentially trouble free operation. Then, once the first operation machining was complete, a second operation machining was performed on the opposite side of the wheel form, this again being done in a conventional manner. After machining both sides, the center of the wheel form was parted-off using a trepanning tool and then recycled. The key technical point observed was that the center makes the rim rigid and supports it in a rigid condition during the entire machining process. The resulting wheel rim displayed all of the advantages of prior art cast rims in one-piece wheels, and included substantially fewer chatter marks, less machining time, fewer scraps, less technician set-up, and a fine machined surface which requires less polishing.